There are two major objectives for this proprosal. The first is to determine if increases in the influx of molecules into the nephron occur following interventions which increase urinary sodium excretion. Using peritubular capillary and tubular microinjection of radioactive tracers, the influx of both neutral and charged molecules will be determined during acute volume expansion and acute hypertension. The second objective is to evaluate the importance of the CNS and gastrointestinal tract in the regulation of sodium excretion. Changes in urinary sodium excretion following selective increases in the plasma sodium concentration perfusing the CNS and various parts of the gastrointestinal tract will be evaluated in the conscious dog with chronically implanted catheters.